This is a continuation of the language of the Remnant which we had in the preceding Psalm. They first address Jehovah, owning that they have a full and glorious anticipation of the victory of their King, and of His establishment in His kingdom, because He had trusted in Him, his God. (See Psa. 18:2-3; Heb. 2:13; 5:7.) Then, in what may be called the second part, beginning with Psa. 21:8, they address Messiah as still in the heavens, but telling Him as it were of His coming victories; and they close by desiring His exaltation, owning Him Lord.
His crown is one of “pure gold” (Psa. 21:3); that is, of unsullied righteousness; and therefore His kingdom such as will last (Heb. 1:8-9); “length of days forever and ever” (Psa. 21:4).
David was the type of the true King thus in victory. And David’s desire was fulfilled (2 Sam. 7:19); as here Messiah’s is in Psalm 21:4.
The 110th Psalm, I may observe, is another instance of a worshipper addressing Jehovah and Christ by turns, as he sees them gloriously seated in the heavens. What characters of communion are our souls entitled to! what discoveries of heaven as it now is does scripture make to us! What sights of glories yet to come do we get there!